LUCENE-9084: circular synchronization wait (potential deadlock) in AnalyzingInfixSuggester

[paulward24]

Issue Report On JIRA

I also reported this at:

https://issues.apache.org/jira/browse/LUCENE-9084#

Bug description

Detailed code (snippets and links) are in the sections after this overview (section Detailed Code and This Patch's Code).

Method ensureOpen() is synchronized (acquires this) and its body contains a synchronized (searcherMgrLock) block (i.e., then acquires searcherMgrLock).

Method ensureOpen() is called two times from public methods add() and update().

A thread calling public methods add() or update() will acquire locks in order:

this -> searcherMgrLock

Public method build() has a synchronized (searcherMgrLock) block in which it calls method add(). Method add(), as described above, calls method ensureOpen().

Therefore, a thread calling public method build() will acquire locks in order:

searcherMgrLock -> this -> searcherMgrLock

2 threads can acquire locks in different order which may cause a circular wait (deadlock).

I do not know which threads call these methods, but there is a lot of synchronization in these methods and in this file, so I think these methods must be called concurrently.

One thread can acquire:

this -> searcherMgrLock (the first order above)

and the other thread can acquire:

searcherMgrLock -> this (the second order above).

Note how the above 2 orders lead to a circular wait.

Detailed Code

Method ensureOpen() is synchronized and its body contains a synchronized (searcherMgrLock):

  private synchronized void ensureOpen() throws IOException { <<<<<<<<<<< see the synchronized keyword
    if (writer == null) {
      if (DirectoryReader.indexExists(dir)) {
        // Already built; open it:
        writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.APPEND));
      } else {
        writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.CREATE));
      }
      synchronized (searcherMgrLock) { <<<<<<<<<<<<<<<<<<<<<<<

https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L371-L379

Method ensureOpen() is called two times from public methods add() and update():

  public void add(BytesRef text, Set<BytesRef> contexts, long weight, BytesRef payload) throws IOException {
    ensureOpen(); <<<<<<<<<<<<<<<<<<<<<<

https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L394-L395

  public void update(BytesRef text, Set<BytesRef> contexts, long weight, BytesRef payload) throws IOException {
    ensureOpen(); <<<<<<<<<<<<<<<<<<<<

https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L406-L407

Public method build() has a synchronized (searcherMgrLock) block in which it calls method add():

  @Override
  public void build(InputIterator iter) throws IOException {
    
    synchronized (searcherMgrLock) { <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
      if (searcherMgr != null) {
        searcherMgr.close();
        searcherMgr = null;
      }

      if (writer != null) {
        writer.close();
        writer = null;
      }

      boolean success = false;
      try {
        // First pass: build a temporary normal Lucene index,
        // just indexing the suggestions as they iterate:
        writer = new IndexWriter(dir,
            getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.CREATE));
        //long t0 = System.nanoTime();

        // TODO: use threads?
        BytesRef text;
        while ((text = iter.next()) != null) {
          BytesRef payload;
          if (iter.hasPayloads()) {
            payload = iter.payload();
          } else {
            payload = null;
          }

          add(text, iter.contexts(), iter.weight(), payload);  <<<<<<<<<<<<<<<<<<<<<

https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L278-L310

Method add() is the same one I linked above.

This Patch's Code

Note that method ensureOpen() (inlined above) is the only place (method or synchronization block) that is synchronized on this.

All the other synchronizations in this file are on searcherMgrLock.

This CR removes the synchronized on this (again, being the only synchronized on this, we can do this change safely).

And moves synchronized (searcherMgrLock) a few lines above, to protect the entire code (that otherwise was protected by synchronized on this).

The above breaks the lock cycle I described earlier.

The fix looks big because it changes indentation.

But only one line is moved by a few lines up.

I.e., from this:

  private synchronized void ensureOpen() throws IOException {
    if (writer == null) {
      if (DirectoryReader.indexExists(dir)) {
        // Already built; open it:
        writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.APPEND));
      } else {
        writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.CREATE));
      }
      synchronized (searcherMgrLock) { <<<<<<<<<<<<<<<<<<<<<< move from here
        SearcherManager oldSearcherMgr = searcherMgr;
        searcherMgr = new SearcherManager(writer, null);
        if (oldSearcherMgr != null) {
          oldSearcherMgr.close();
        }
      }
    }
  }

To this:

  private void ensureOpen() throws IOException { <<<<<<<<<<<<<<<<<< remove synchronized --- can lead to circular wait --- and legal to remove
    synchronized (searcherMgrLock) { <<<<<<<<<<<<<<<<<<<<< move to here
      if (writer == null) {
        if (DirectoryReader.indexExists(dir)) {
          // Already built; open it:
          writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.APPEND));
        } else {
          writer = new IndexWriter(dir, getIndexWriterConfig(getGramAnalyzer(), IndexWriterConfig.OpenMode.CREATE));
        }

        SearcherManager oldSearcherMgr = searcherMgr;
        searcherMgr = new SearcherManager(writer, null);
        if (oldSearcherMgr != null) {
          oldSearcherMgr.close();
        }
      }
    }
  }

Here are all the places where synchronized (searcherMgrLock) appears in this file (and again, no other synchronized on other objects is done):

• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L281-L332
• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L379-L385
• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L654-L657
• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L870-L873
• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L898-L901
• https://github.com/apache/lucene-solr/blob/master/lucene/suggest/src/java/org/apache/lucene/search/suggest/analyzing/AnalyzingInfixSuggester.java#L926-L929

I.e., doing the synchronization like above is safe and consistent with the rest of the file.

[mikemccand]

It looks like this one was indeed merged -- closing the PR. Thank you @paulward24!